This invention relates to improved methods and apparatus for evaluating the condition of subsurface oilfield piping and, more particularly, relates to magnetic, induction type apparatus for nondestructive testing of tubing and casing.
It is common knowledge in the petroleum industry to use steel or iron pipe in nearly all oil and gas wells. Such piping serves useful purposes including shutting off water bearing formations, preventing formation deterioration and shutting off intermediate oil or gas zones when it is desired to drill deeper. During the lifetime of a well a variety of conditions may result in the deterioration of such subsurface piping, including pits, cracks, holes, thin walls, structure changes in the metal and the like. Such deterioration may result from various causes. Electrochemical theory postulates a tendency of steel, or other materials, in an electrolytes environment, such as subsurface formations, to go into solution causing corrosive deterioration. Likewise, during drilling operations drill pipe collars may rub on the inside wall of the pipe causing excessive wear.
There have been various proposals for the design of equipment to measure pipe anomalies while the piping is still in place. One such system which has been widely accepted by the industry is described in U.S. Pat. No. 3,543,144, issued to W. T. Walters et al on Nov. 24, 1970. This system includes an elongated magnetizer assembly including a pair of elongated pole pieces, having a diameter only slightly less than the inside diameter of the piping, and a central core having a magnetizing winding thereon between the pole pieces. A plurality of detectors are positioned between the pole pieces into contact with the interior surface of the casing. A high-intensity, undirectional magnetic field emits from the magnetizing winding with the object of saturating the ferrous piping. If there is no defect in the piping, the magnetic flux lines pass undisturbed through the piping between the pole pieces. When a defect in the casing exists a portion of the magnetic field will "leak" out the piping and flow about the defect. This flow is detected within the wall contact detectors.
For detection of magnetic flux lines, each shoe includes two coils which detect flux leakage. In addition, each shoe includes two coils to detect eddy current flow. When a flux leakage coil detects flux leakage the companion eddy current coil generates a signal if the defect is inside the casing wall. Electrical signals of the greatest signal from the shoes in the upper ring and the greatest the signal from the shoes of the lower ring are generated and transmitted to the surface. A third recording indicates whether the defect is internal. Lastly, all signals from the upper ring are processed to yield an average measurement of the circumferential extent of the defect.
While the system described has found acceptance in nondestructive testing of subsurface piping it has proven less than desirable in indicating the nature of anomalous response obtained. All tool responses must be treated as corrosive unless prior knowledge of the well indicates otherwise. Thus, to interpret subsurface signals an operator is required to employ actual records of the well indicating the presence of equipment such as centralizers and scratchers. Otherwise all signal responses are treated as a defect. This leads to problems, particularly if well records are incomplete or inaccurate.
These and other disadvantages are overcome with the present invention by providing an apparatus for nondestructive testing of well casings which does not require prior knowledge of the well and provides a plurality of quantative indications of casing condition.